The present invention pertains to polymers prepared from substituted pyrazines containing at least one substituent group having a hydrogen atom attached to a carbon atom attached to the ring, aromatic polyaldehydes and aldehydes or a nitrogen-containing heterocyclic compound containing at least one substituent group having a hydrogen atom attached to a carbon atom attached to the ring and a polymerizable unsaturated group.
A copending application Ser. No. 670,428 entitled "Thermosettable Polymers or Prepolymers Prepared from Polymethylated Pyrazines and Aromatic Polyaldehydes and Cured Products Therefrom" filed Nov. 9, 1984 by Duane S. Treybig and Loren L. Swearingen discloses thermosettable polymers prepared by reacting an alkyl substituted pyrazine with a dialdehyde. The cured prepolymer has excellent properties at elevated temperatures as well as a high char yield (61% to 71%) in nitrogen. These properties of the resin are particularly suitable for the preparation of graphite fiber composites or other cured articles which may be subjected to fire. However, these thermosettable prepolymers are cured by a condensation reaction which generates water. This water vaporizes during cure and produces undesirable voids and/or surface imperfections in the composite or other cured article.
These prepolymers can be terminated with an ethenyl (vinyl) substituted mono-aldehyde or a vinyl and alkyl substituted aromatic heterocyclic compound. The resulting vinyl terminated polystyrylpyrazine prepolymers can then be cured by an addition reaction via the unsaturated terminal groups, thereby eliminating the release of water vapor and resulting in composites or other cured articles which do not possess surface imperfections.
The vinyl terminated polystyrylpyrazine prepolymers of the present invention are also suitable as novel comonomers for reaction with N,N'-bis-imide resins. The copolymerization of the vinyl terminated prepolymers with N,N'-bis-imide resins occurs at a lower cure temperature than that of a maleimide resin alone, which results in energy conservation. Since the copolymerization is an addition reaction, volatile generation is minimized. The copolymers are more amorphous, less brittle, less crystalline and tougher polymers than N,N'-bis-imide polymers. Depending upon the choice of the vinyl termination molecule, the char yield of the resultant copolymer is higher than the char yield of the cured N,N'-bis-imide on pyrolysis. As a consequence of the high char yield of these copolymers, they exhibit better fire resistance than the cured N,N'-bis-imides. Char yield is defined herein as the percent by weight of the polymer remaining after exposure to 950.degree. C. during a thermogravimetric analysis of the polymer in a nitrogen atmosphere.